This invention relates to a latent heat storage apparatus and a latent heat storage solution therefor for storing and retrieving a cold heat substantially in or from a latent heat thereof necessary for air conditioning, cooling, refrigerating, and so forth.
Aqueous solutions of ethylene glycol, propylene glycol, calcium chloride, sodium chloride, or the like have been used as cold heat transfer solution, or as so called brine in an evaporation tank of refrigeration cycle system to absorb or evolve the cold heat to a temperature below 0 degree.
If the load which receives the heat with brine of this kind requires a temperature range within strict limits, an ample volume of brine sufficient to moderate the unavoidable temperature fluctuation has to be prepared to keep the temperature constant in the evaporation tank and also at the load site, because the brine or brines store or transfer the cold heat by way of heat capacity of the solution, which heat is referred to hereinafter as kinetic heat which is the opposite of latent heat.
Therefore, a brine tank or the evaporation tank for the system relying on the kinetic heat becomes large in scale to contain the ample brine volume, which is not economic due to increase of the initial installation and running costs.
A technique for latent heat storage apparatus, therefore, has been proposed nowadays to resolve the above problem, wherein a part of the brines is subjected to freezing and thawing so that latent heat is applied to absorb/evolve cold heat. The means for storage of cold heat provides a high density of such heat compared to the means relied on for the kinetic heat, and also an easy evolution of cold heat within a stable range of temperature. The means for cold heat is assembled in a simple and compact system which is widely applied in a field of civil engineering including the food industry.
The latent heat storage apparatus includes two main systems, of which one is so called a capsule system, and other is so called an ice bank system. The former is furnished with a number of capsules piled in a cold heat storage tank, in which capsules enclose inorganic salt solution coinciding with the eutectic composition and the capsules contacting the brine absorb/evolve the cold heat as latent heat. The latter comprises a cold heat storage tank containing brine of inorganic salt or ethylene glycol water solution which is frozen to store cold heat as the latent heat with a cooling tube, wherein the cold heat is retrieved from the frozen solid which evolves the latent heat of fusion.
In the known system, however, having interposed the capsule, the cold heat is transferred in a multistage manner through the cooling tube of the refrigerator, brine and capsule, with the result that the evaporation temperature of the coolant in the refrigerator cooling tube has to be set low so as to reduce the thermal efficiency of the system.
As the capsule is generally formed in ball or cylindrical shape, the capsules piled in the storage tank provide a number of spaces between one another, which, together with some thickness of the wall, inevitably reduce the volumetric density of the latent heat storage solution per unit volume of the storage tank.
Further, the capsule is formed of plastic resin to readily enclose the latent heat storage solution, and thereby to save production cost. The resin wall is thermally less conductive than that of metals. The poor conductivity of the wall in addition to the wall thickness results in a power consumption increase for the refrigerator and in a prolonged processing time consequence to a low operational temperature of refrigerator coolant contacting the capsule.
In the reverse process for retrieving cold heat, the system having disposed the capsule in the thermal passage has to set the storage temperature much lower than the desired retrieving temperature because of the above-mentioned reason, resulting in a less efficient system.
As for the ice bank system, a technique is disclosed in Japanese Laid Open Application No. 62-62192. That document proposes a system furnished with an evaporator comprising a cooling cycle as a heat exchanger in a heat storage tank in which coolant is filled up. The coolant circulates to and from a load. The coolant as a latent heat storage solution is made of two-component inorganic salts solution each of which has an eutectic point below 0 degree respectively, more practically, an aqueous solution of potassium chloride and sodium chloride each of which forms an eutectic with ice and respective salt.
Further, a technique is disclosed in Japanese Laid Open Application No. 2-214793 which proposes to improve further the latent heat storage solution which revolves cold heat at -5 degree during the retrieving process from the latent heat of fusion thereof, which is made of an aqueous solution of potassium nitrate and sodium nitrate.
The first mentioned Japanese document, however, simply refers each of eutectic points for the water-potassium chloride, and the water-sodium chloride, i.e., the binary eutectic points. Because no further technique has been disclosed for the three-component system nor a ternary eutectic point, that document teaches how to retrieve the desired cold heat at a concentration of the solution, without ability to control accurately the temperature in advance.
When the initial concentration of salts in the latent heat storage solution exceeds that of the ternary eutectic, the inorganic salt crystal or salt hydrate separates to deposit in the liquid mixture upon cooling. The salt deposited at the bottom of the heat storage tank is hard to dissolve again in the solution upon heating to evolve the cold heat, due in part to the low temperature of the solution.
In the second mentioned Japanese document, the latent heat storage solution consists of potassium nitrate and sodium nitrate at a specific concentration, wherein the retrievable cold heat is limited at about -5 degrees. The technique further involves a problem as described in the first mentioned Japanese document to separate the nitrate salt upon cooling of the solution, because the concentration range of potassium nitrate includes a range beyond that of the ternary eutectic.
It is a primary object of this invention to provide a latent heat storage apparatus so formed in as to be retrievable a specific cold heat below a temperature of 0 degree almost invariably, in the manner similar to the so called ice bank system.
It is another object of this invention to provide a latent heat storage apparatus and a latent heat storage solution therefor so formed as to be retrievable invariably a cold heat in which temperature is arbitrarily specified easily within a certain range.
The present invention is applicable to a latent heat storage apparatus having a heat storage tank containing latent heat storage solution consisting of a plurality of inorganic salts dissolved in an aqueous solution, wherein cold heat is retrievable from the latent heat storage solution using the latent heat thereof.
A feature of this invention is that the latent heat storage solution is formed as an aqueous solution of N-component mixture of (N-1) kinds of inorganic salts, in which N is at least greater than 3, wherein the concentration of the salts is set in an eutectic tortuous plane including the ice point of liquid-solid equilibrium diagram, wherein the cold heat is stored invariably at an arbitrary temperature in the latent heat storage solution as the latent heat thereof to form ice, binary eutectics with ice, or separated substances in advance, whereby the cold heat is retrievable invariably at the arbitrary temperature in a range above the N-component eutectic point and below any one of binary eutectic points of ice and salts.
It is more preferable to set the concentration of inorganic salts in the solution within a range of concentration 50 to 98 wt. % of that of any one of eutectic lines connecting the ice point and the binary eutectic points with ice and thereof.
It is preferable as well to set the concentration of any one of inorganic salts in a range of 60 to 98 wt. % of that of the binary eutectic point with ice and thereof, and the total concentration of the solution is less than that of the N-component eutectic point.
The apparatus, therefore, achieves the object of the invention by inclusion in a system hereinafter described to store or retrieve the cold heat in or out of the latent heat storage solution by applying the binary eutectics and N-component eutectic included in the N-component tortuous plane.
It is practical to form the system such that the apparatus is connected with the load through circulating tubes to retrieve the cold heat directly from the separated substances which have stored that heat as latent heats thereof, and thereby achieves a preferable thermal efficiency.
The apparatus may be generally formed to dispose a heat exchanger in the latent heat storage tank, wherein coolant circulates in the heat exchanger to work for an evaporator, but the invention is not limited thereto.
The function and effects of the present invention will be described as follow.
As described earlier, the latent heat storage solution of this invention consists of an aqueous solution dissolving at least two inorganic salts which forms N-component solution wherein N is greater than 3(N.ltoreq.3). The concentration of inorganic salts in the latent heat storage solution is set in a range corresponding to that of the ice point, binary eutectic points of the salts with ice, and the N-component eutectic point. If the latent heat storage solution is cooled below the ice point as illustrated in FIG. 2, the solution, which maintains liquid phase for a while because of a depression of freezing point of aqueous solution, commences to separate ice and (N-1) kinds of binary eutectics in the order of higher eutectic point at a definite temperature, namely the freezing point, depending on the composition of the mixture. The solution becomes a two-phase solution consisting of solid (ice and binary eutectics), and liquid (a condensed aqueous solution). As further cooling of the mixture proceeds, the storage solution commences to separate the N-component eutectic until finally a single phase of solid mixture of ice, binary eutectics and N-component eutectic is formed.
The separation process requires a latent heat. The latent heat storage solution absorbs cold heat equivalent to a solidified latent heat of water to form ice, and each one of solidifying latent heats of eutectics to form the corresponding binary eutectics and N-component eutectics.
In the process, therefore, the temperature changes slower compared with that of the single component of water, or even remains unchanged especially during the separation of N-component eutectic at the definite N-component eutectic point.
If, in reverse, the solid mixture of ice and eutectics is heated until it liquefies completely to reach the depressed freezing point, the latent heat storage solution retrieves the cold heat equivalent to the heat of fusion, the reverse of the solidifying latent heat. On further heating, the temperature of the liquid solution is elevated, thereby retrieving the cold heat equivalent to the kinetic heat at the rate of the specific heat of the solution.
Therefore, on comparing the kinetic heat and the latent heat with respect to a unit weight, the latter is generally much greater than the former. In the state in which both solid and liquid exist, that is the state of ice+eutectics+liquid solution, the cold heat is retrievable from the latent heat of fusion which ice and eutectics release.
Further, because the N-component eutectic point is lower than every binary eutectic point, it is possible to set arbitrarily the temperature to retrieve the cold heat within a range between any one of the freezing points and the N-component eutectic point by suitable preparation of the composition and the initial concentration of the solution of inorganic salts.
And further, it is possible to maintain a ratio of ice and eutectics to the aqueous solution, or an ice packing factor (IPF in short, hereinafter) of more than 30%, because the concentration of the inorganic salts in the latent heat storage solution is set in the range of 50 to 98 wt. % of that equivalent to the eutectic line connecting any one of the binary eutectic points with water and the N-component eutectic point.
It is possible as well to avoid separation of the crystal salts or hydrates thereof during the cold heat storage process, because the concentration of any one of inorganic salts is set in a range of 60 to 98 wt. % of that of the binary eutectic point with ice and thereof, and the total concentration of the solution is less than that of the N-component eutectic point, whereby one need not worry about the dissolving speed of the crystal salts and hydrates in the cold heat retrieval process, because there is no such solid which may pile at the bottom of the heat storage tank.
Because the range of concentration of the salts in the latent heat storage solution is set, the solid-liquid phase tortuous plane including the ice point and the N-component eutectic point shows a gradual slope compared with a steep slope of the remaining solid-liquid tortuous planes which include the melting points of any couple of inorganic salts and the N-component eutectic point. Thus, the cold heat can be efficiently stored or evolved invariably at any temperature within the range between the depressed freezing point and the N-component eutectic point depending on the composition and initial concentration, thereby maintaining the maximum IPF at the desired ratio.
Comparing the slope of solid-liquid line connecting the ice point and each of the binary eutectic points with that of lines connecting the binary eutectic point next to the ice point and the N-component eutectic point at the rather higher part below the ice point, the latter are in most cases more gradual than the former. In using the latent heat of water alone in the two-component system, the cooling heat is advantageously retrievable invariably by forming the ice+binary eutectics with ice within the appropriate IPF.
In this invention, therefore, it is possible to supply the cold heat to the load at the freezing point as desired within the temperature range below the binary eutectic point with ice above the N-component eutectic point, in which the freezing point is arbitrarily set with the composition and concentration of the (N-1) kinds of inorganic salts of the latent heat storage solution as to the N-component aqueous solution filled in the heat storage tank, wherein N is greater than three (N.gtoreq.3).
In this present invention, because the cold heat is stored in or retrieved from the latent heat storage solution utilizing the binary and N-component eutectics in the N-component tortuous plane which enables arbitrary selection of the freezing or fusion points, the cold heat is stably retrievable with almost invariable temperature.
It is further possible in this invention to save a loss of thermal energy, and to raise the thermal efficiency of the latent heat storage apparatus, because the latent heat of separated substances, the binary or N-component eutectics, is directly exchangeable to the cold heat at the load site.
A solid-liquid line also represents a solubility curve corresponding to respective temperatures. Therefore, concentration of the binary eutectics in the N-component tortuous plane, in which temperature is above N-component eutectic point, is smaller than the saturated concentration, that is, that of N-component eutectic. During the retrieval process of cold heat from the latent heat, the binary eutectics can dissolve in the liquid at a faster rate without depressing the rate of the retrieval process. The faster dissolving rate, therefore, enables a much longer duration for at an invariable fusion point, because the eutectics dissolve without a time lag corresponding to the rate of process without raising the fusion point.